/Engine/Plugins/Runtime/GeometryProcessing/Source/DynamicMesh/Public/FaceGroupUtil.h
Collect triangles by group id. Returns array of triangle lists (stored as arrays). This requires 2 passes over Mesh, but each pass is linear
namespace FaceGroupUtil { void FaceGroupUtil::FindTriangleSetsByGroup ( const [FDynamicMesh3](API\Runtime\GeometryCore\DynamicMesh\FDynamicMesh3) & Mesh, [TArray](API\Runtime\Core\Containers\TArray)< [TArray](API\Runtime\Core\Containers\TArray)< int32 >> & GroupTrisOut, int32 IgnoreGID ) }